This application is based on Japanese Patent Application Nos. 2001-229598 filed on Jul. 30, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to an apparatus for controlling an automatic transmission for an automotive vehicle, and more particularly to techniques for minimizing a shifting shock of the automatic transmission upon a shift-down action of the automatic transmission while an accelerator pedal of the vehicle is in an operated state.
2. Discussion of Related Art
For an automotive vehicle, there is known an automatic transmission which has a plurality of hydraulically operated frictional coupling devices operable with pressurized fluid supplied from accumulators and which is shifted by engaging and releasing actions of selected ones of those frictional coupling devices. There is widely known an automatic transmission control apparatus arranged to control a hydraulic engaging pressure of such a hydraulically operated frictional coupling device such that the hydraulic engaging pressure is changed at a controlled low rate, so as to reduce or minimize a shifting shock of the automatic transmission. JP-A-1-150049 discloses an example of such an automatic transmission control apparatus, which is arranged to control shift-down actions of the automatic transmission, by learning compensation of a back-pressure of the accumulator for a frictional coupling device associated with a shift-down action of the transmission. This control apparatus is adapted to effect the learning compensation of the back-pressure of the accumulator, depending upon a shifting time of the automatic transmission, so as to effectively reduce the shifting shock, irrespective of a variation in the actual friction coefficient of the specific frictional coupling device with respect to a nominal value, or a chronological change in the friction coefficient during use of the frictional coupling device.
Such a vehicle automatic transmission control apparatus arranged to control the back pressure of the accumulator on the basis of the shifting time of the automatic transmission does not permit the accumulator to perform the intended function of reducing the rate of change of, or slowing changing the hydraulic engaging pressure of the corresponding frictional coupling device, where the shifting time has a large amount of variation that may cause the accumulator to be inoperable. In this case, the automatic transmission operated under the control of the control apparatus undesirably suffers from a large shifting shock.
The present invention was made in the light of the background art discussed above. It is therefore an object of the present invention to provide an apparatus for controlling a vehicle automatic transmission, which apparatus permits effective reduction of a shifting shock of an automatic transmission, irrespective of a variation in the shifting time of the transmission.
The object indicated above may be achieved according to the principle of this invention, which provides an apparatus for controlling a vehicle automatic transmission having a hydraulically operated frictional coupling device operable to effect a shifting action of the automatic transmission, and an accumulator connected to the frictional coupling device, said apparatus comprising: (a) accumulator-back-pressure learning compensation means for effecting learning compensation of a back pressure of the accumulator such that an actual value of a rate of change of a rotating speed of a selected one of rotary elements of the automatic transmission, which rotating speed changes as a result of the shifting action, approaches a predetermined target value; and (b) accumulator-back-pressure control means for controlling the back pressure of the accumulator, on the basis of an output of the accumulator-back-pressure leaning compensation means.
In the automatic transmission control apparatus of the present invention constructed as described above, the accumulator-back-pressure learning compensation means is arranged to effect the learning compensation of the back pressure of the accumulator such that the actual value of the rate of change of the rotating speed of the selected rotary element approaches the predetermined target value, so that the shifting shock of the automatic transmission can be more effectively reduced irrespective of a variation in the shifting time of the automatic transmission, than in the prior art in which the learning compensation of the back pressure of the accumulator is based on the shifting time. The present automatic transmission control apparatus prevents the accumulator from being inoperable, even where the shifting time of the automatic transmission has a large amount of variation, thus enabling the accumulator to perform the intended function of reducing the rate of change of the hydraulic engaging pressure of the frictional coupling device, to thereby effectively reduce the shifting shock of the automatic transmission.
According to a first preferred form of the present invention, the accumulator-back-pressure learning compensation means effects the learning compensation of the back pressure of the accumulator such that an actual value of an inertia-phase-start time approaches a predetermined second target value while the actual value of the rate of change approaches the predetermined first target value. The inertia-phase-start time is defined as a period of time from a moment at which the automatic transmission is commanded to effect the shifting action, to a moment of initiation of a change of the rotating speed of the selected rotary element as a result of the shifting action. This arrangement is effective to prevent the accumulator from being inoperable due to an excessively large amount of variation in the shifting time of the automatic transmission, and assures an improved stability in the function of the accumulator of reducing the rate of change of the hydraulic engaging pressure of the frictional coupling device, so as to effectively reduce the shifting shock of the automatic transmission.
According to a second preferred form of this invention, the shifting action of the automatic transmission is effected by a releasing action of the hydraulically operated frictional coupling device and an engaging action of a one-way clutch, and the accumulator-back-pressure learning compensation means effects the learning compensation of the back pressure of the accumulator such that the one-way clutch is engaged while a pressurized fluid is fed into an accumulator chamber of the accumulator. This arrangement is effective to reduce the shifting shock of the transmission upon engagement of the one-way clutch.
According to a third preferred form of this invention, the automatic transmission is adapted to a front-engine front-drive vehicle, and includes a main shifting portion disposed on an input shaft thereof, and an under drive portion disposed on a counter shaft which is spaced from and parallel to the input shaft, and the shifting action is a shift-down action which takes place in the under drive portion, the selected rotary element consisting of an input member of the under drive portion. This arrangement is effective to reduce the shifting shock of the automatic transmission which is arranged for use on the FF vehicle and which includes the under drive portion disposed on the counter shaft parallel to the input shaft on which the main shifting portion is disposed.
In one advantageous arrangement of the above-indicated third preferred form of this invention, the under drive portion of the automatic transmission includes a clutch which is provided as the hydraulically operated frictional coupling device and which is released to effect the shift-down action, and the accumulator is connected to the clutch to slowly change a hydraulic engaging pressure of the clutch in the process of a releasing action of the clutch. This arrangement is effective to reduce the shifting shock of the automatic transmission upon the shift-down action involving the releasing action of the clutch, irrespective of the shifting time of the transmission.
In the automatic transmission control apparatus according to the above-indicated advantageous arrangement of the third preferred form of the invention, the accumulator-back-pressure learning compensation means may be arranged to effect the learning compensation of the back pressure of the accumulator such that the back pressure is lowered when the rate of change of the rotating speed of the input member of the under drive portion of the automatic transmission is lower than a predetermined target value in the process of the shift-down action which takes place during an operation of an accelerator pedal provided on the front-engine front-drive vehicle. This arrangement is effective to reducing the shifting shock upon the shift-down action, irrespective of a variation of the shifting time.